Copy an idea once, and it's plagiarism; copy it a dozen times, and it's a cliché; keep on copying, and sooner or later you've got yourself a genre – and at that point, the fact that you've copied an idea matters less than any tiny variations you manage to introduce. In Away We Go, the main characters travel around America, meeting eccentric or plain screwed-up people and discovering heart-warming truths about the importance of family, and I can't decide where it stands on the continuum I have described. My head says I should be damning Dave Eggers and Vendela Vida, the husband-and-wife writers, for the lazy and derivative format (cf Little Miss Sunshine, The Straight Story, etc); but my soft heart is urging me to praise them for importing a little originality into it.

We open with Burt and Verona (John Krasinski and Maya Rudolph) in bed, his head busy under the quilt: why, he wonders, does her vagina taste so fruity? Like quite a lot of the film, that bit of dialogue is cuter on screen than it reads on the page, thanks largely to the warmth that Krasinski and Rudolph bring to it. It's also a neatly conceived way of announcing both Verona's pregnancy, the mainspring of the plot, and the sort of film we're in for – wacky, talky, uninhibited, sexy, domestic (later on, "melancholy" gets added to the list).

Move on six months or so: over dinner, Burt's parents (Jeff Daniels and Catherine O'Hara) announce that they won't be around for the baby's birth because they're finally going to fulfil their long-cherished dream of going to live in "the City of Light" – Antwerp (I think I mentioned "wacky" earlier). Since Maya and Burt are only living in this unspecified but cold and remote part of America because his parents are there, and since their jobs are portable – he sells insurance futures, she does detailed and gruesome anatomical drawings for medical textbooks (wacky!) – they decide they could just as easily live anywhere, and start looking. And that's about it, as far as story goes. Cesare Pavese wrote that, "if immoral works of literature exist, they are works in which there is no plot." If we're applying that to films, then Away We Go is practically depraved.

First, so the intertitles announce, "Away to Phoenix". At the airport (at least this isn't a road-movie), Verona's old boss, Lily, greets her with encouraging cries of "You're huge! And your face is so fat!" before she, along with her misanthropic husband and understandably sullen children, treats them to a masterclass in dysfunctional marriage and parenting – Allison Janney delivering her excruciating lines with a jolting energy that makes everything that comes after seem slightly dreary. Next, by rented car 120 miles to Tucson, to see Verona's younger sister, and a somewhat sentimental interlude (but in a bathroom showroom! sitting in a bathtub!) in which it is revealed that Verona can never bring herself to talk about their dead parents.

From Tucson they travel north by train, having been refused permission to fly because Verona looks way more than six months pregnant (a running joke about her vastness and people's readiness to comment on it is the closest thing the film has to a coherent structure). In Madison, Wisconsin, Burt has an interview for a job promotion and they plan to see his childhood friend Ellen: she has grown up into Maggie Gyllenhaal, changed her name to LN, and is living in a dippy, offensively self-righteous household based on the "continuum" principle of keeping your children close at all times, and adopting the gender roles of "the seahorse community". The episode is very funny, and has a ring of truth, but at this point the film's own self-righteousness starts to weigh a little: it seems as though Burt and Verona are going to turn out to be the only sane, unselfish people left in the world, and the film's whole trajectory is towards a confirmation of their loveliness.

Away to Montreal, where their friends Tom and Munch seem to have something approaching a perfect life: a gorgeous bunch of kids in a lovely house, even if they do always turn off The Sound of Music before the end so that the children don't have to deal with the whole Nazi thing. Tom and Munch even manage to go out at night and have fun. Ah, but there's a worm in the bud: the children are adopted; Munch keeps having miscarriages, and their marriage is full of sadness.

A lightning change of direction: away to Miami to rescue Burt's brother, whose wife has walked out on him (he wonders if it is OK for him to tell their daughter she has been murdered).

The final section is entitled "Home": Burt and Verona drive their beaten-up old Volvo to her family house, somewhere down south, where she hasn't been since her parents died years earlier; and this beautiful old building, set among woods by a lake, is where they will bring up their child. After all that unconventionality, all that picaresque and zany humour, it turns out that this is really a deeply orthodox American morality tale about the importance of parents, of talking about your feelings, of coming to terms with the past, and how there's no place like home.

Away We Go is unconventional and cynical only as far as that looks cute; it certainly doesn't want to rock any boats or point any fingers.

This isn't a complaint, though, or not much of one: it's nice to see Sam Mendes trying his hand at a happy couple, rather than the dry, loveless marriages depicted in American Beauty and Revolutionary Road. Away We Go is slight, implausible and more fuddy-duddy than it wants to let on, but it's well-made, interesting, cynical enough to undercut its moments of sentimentality, and kind of fun. When I came out, I felt a little more cheerful than when I went in.